The proposed research is a study to elucidate the origin and nature of the centrifugal fibers of the eighth cranial nerve. Its primary aim is to determine whether or not those centrifugal fibers called the "olivocochlear bundle" are really autonomic in nature and if the efferent terminals in the organ of Corti are but branches of the spiral ganglion cells. The study is being carried out utilizing 1) an autoradiographic technique for labeling terminals or entire neurons of small neuronal pools through somal uptake of tritiated amino acids and transport of synthesized proteins into all neuronal parts; and 2) a histochemical neuron labeling method involving horseradish peroxidase which stains the unmyelinated but not the myelinated neurons. Both the labeled amino acids and the horseradish peroxidase are applied directly to cochlear structures in micron 1 quantities by use of microsurgical techniques. Processed tissues will be studied both light and electron microscopically. Findings should help to resolve the question of the source of the efferent terminals in the origin of Corti and should further clarify the extent of HRP uptake by inner ear structures.